1. Field of Invention
The present invention generally relates to a compact-size or narrow gauge linear motion rolling guide unit including a trackrail member and a casing straddling said track rail member via plurality of rolling balls which are made to move infinitely between the two members so that the linear motion guide unit provides linear motion having stabilized precision and is featured by high rigidity in structure under such a composite load as including upwardly, downwardly and laterally directioned loads, momentary load etc, therefore the present linear motion rolling guide unit can be preferably used in various industrial field such as machine tools operated under vibration and impact, robotic machines requiring high precision workings, various manufacturing machinery and so on.
2. Description of the Prior Art
A linear motion rolling guide unit is known in the art. Japanese Patent Application No. 59-109573 (FIG. 5) that was filed by the present applicant discloses a linear motion rolling guide unit comprising a track rail member 1' having a generally square cross section, a slidable casing 2' having an inverted U-shaped cross section and straddling said track rail member via a plurality of balls in freely movable relation therewith, side panels fixed onto the longitudinally opposing ends, respectively, of said slidable casing, said track rail member having a pair of guide grooves for balls on the opposite side surfaces, each extending in parallel with the longitudinal axis of said track rail member, said slidable casing 2' having a pair of guide grooves for balls in mating relation with a pair of said track rail side guide grooves so that one of the track rail side guide grooves and the mating casing side guide groove form a load bearing passage for balls between said track rail member 1' and said casing 2', said slidable casing comprising a pair of ball return passages 7' there-within, each extending in parallel with said load bearing casing side guide groove, each of said side panels comprising a pair of curved direction changing passages 5' each adapted to connect one end of the casing side guide groove with the adjacent end of the ball return passage so that together with the direction changing passage of the opposite end side panel, an endless ball circulating path is formed, wherein in each of said side panels, each of said direction changing passages is so positioned as seen on a cross section of the slidable casing that the related ball return passage may be positioned in the casing upwardly and inwardly relative to the ball located at its casing side guide groove.
With this novel arrangement of ball return passages in the casing, an appreciable technical merit was obtained such that the breadth of the casing can be markedly reduced so that the unit provided with such a compact-size casing can be used even in such a narrow space that any of linear motion rolling guide units of conventional type could not be used due to a greater casing size.
However, even the above-mentioned novel guide unit has the following disadvantages:
Namely, the length of the curved direction changing passage in the side panel becomes undesirably greater so that a greater resistance takes place when balls travel through the direction changing passage.
Furthermore, the curved configuration to be formed for direction changing passage in the side panel is complicated and further, due to the lengthened direction changing passage, the number of balls simultaneously traveling through the direction changing passage increases, thus resulting in an increased manufacturing costs.
Referring to FIG. 5, illustrating a cross section of the casing of the prior art unit, wherein the ball return passage is located inwardly toward the longitudinal axis of the casing by a distance l relative to the perpendicular line passing through the center of the ball present in the load bearing passage. In practice, a plurality of tool mounting holes 6 must be provided in the casing at the locations sufficiently apart from the inner surface thereof, therefore, even if the above-mentioned distance l is made greater, the width of the casing cannot be greatly reduced so far as tool mounting holes 6 must be provided as mentioned above.
In the light of the situation was proposed in the construction of casing that to reduce the length of the direction changing passage (refer to FIG. 6 illustrating another prior art unit), ball return passage 7", as seen in a cross section of casing 2", is located above and outwardly relative to the ball 10" located in the load bearing passage. If, in this case, the ball return passage 7" is located at an angle of 45.degree. outwardly relative to the perpendicular line passing through the center of the load bearing ball 10", thereby shortening the length of the direction changing passage, some of the disadvantages inherent in the above-mentioned prior art unit may be eliminated but the arcuate section of the direction changing passage along and through which the balls located in the load bearing passage are travelled in a curved path (the arcuate section defined, in FIG. 6(a), between the two dotted lines indicated by arrows A and B, respectively, which substantially corresponds to half length of the direction changing passage) has too short a distance to allow the balls making smooth direction changing movement, so that as shown in FIG. 6(b) where part of FIG. 6(a) is seen from the direction of arrow A, the length of the direction changing passage is too short as shown in FIG. 6(b) so that the said arcuate section, viz., half length of the direction changing passage must communicate with the adjoining arcuate section, viz., another half of said passage at the point to provide a length of the direction changing passage shorter by a distance corresponding to the arcuate section defined by angle .theta. than the circumferential distance of a complete semicircle.
Namely, the direction changing passage cannot form a complete semicircle, since each of the halves of the direction changing passage does not form a precisely quarter of a circle, so that the load bearing guide groove side half and the ball return passage side half of the direction changing passage cannot communicate tangentially with each other, thereby to give rise to rolling resistance between the balls and the inner surface of the direction changing passage when the balls are travelling therethrough.